Takamatsu receives 1,150.1 mm; Kochi, 2,666.4 mm
Japan's meteorological agency gives 1991–2020 average annual precipitation of 1,150.1 mm at Takamatsu and 2,666.4 mm at Kochi. These are station figures, not prefectural averages. The agency explains that moist air from the south rises over the Shikoku Mountains and brings rain to the Pacific side, while the Seto Inland Sea side is often drier in the mountains' rain shadow.
Both stations receive more rain in June and July's rainy season and in September's autumn rains and typhoons. For farming, both the yearly total and the availability of water when crops need it matter.
Kagawa divided stored water by time
According to the agriculture ministry, Kagawa's rivers are short and steep, so rainwater can quickly reach the sea. Its many reservoirs hold some of that water back. Manno Pond is a prominent example, repeatedly repaired over centuries to supply farmland.
Allocating stored water required rules as well as infrastructure. In the 'mizu-buni' practice described by the ministry, allocations could reflect field area and access time could be measured by burning incense. Delivering water downstream depended on agreements about who could draw it and when.
Rainy Kochi also needed canals
Kochi Prefecture says the Yamada-seki canal on the right bank of the Monobe River irrigates about 2,000 hectares. An intake structure and roughly 26 km of main canal were built in response to chronic irrigation shortages. They helped bring river water steadily to the fields that needed it.
The prefecture's Monobe River conservation plan also identifies low flows and stretches that run dry, alongside agricultural water withdrawals. Water delivered to fields and water left in the river both require attention.
Kagawa's ponds and Kochi's canals both move water to the right place at the right time. Following the facilities behind the rainfall figures reveals the continuing work of storage, allocation and maintenance.